1. Field of Invention
The present invention relates to a method for manufacturing a semiconductor device. More particularly, the present invention relates to a method for manufacturing a metal plug.
2. Description of Related Art
In general, a metal plug is formed by first etching an insulation layer to form an opening and then forming a conformal barrier layer over the insulation layer and the sidewalls of the opening. Next, metal is deposited to fill the opening and cover the barrier layer. Finally, that portion of the metallic layer above the barrier layer is removed so that a metal plug is formed inside the opening.
There are several methods to remove the metallic material above the insulation layer. For example, one method of forming a tungsten plug is to etch back a portion of the tungsten layer using fluoride-containing plasma. However, due to the high reactivity of fluoride plasma, the upper surface of the tungsten plug may cave in a little. Therefore, a difference in height of up to about 1000 .ANG. between the tungsten plug surface and its neighboring barrier layer is possible.
A second method is to polish the barrier layer using a chemical-mechanical polishing (CMP) operation after the tungsten layer is etched back so that the degree of concavity on the upper surface of the tungsten plug is reduced. However, the barrier layer of a tungsten plug is usually a titanium nitride layer while the insulation layer is normally a silicon oxide layer. Since titanium nitride and silicon oxide are materials having similar polishing rates in a CMP operation, a portion of the oxide insulation layer on each side of the tungsten plug will also be removed after the titanium nitride barrier layer is removed. In other words, the tungsten plug will protrude above the oxide insulation layer after the polishing operation, and often can be as high as 1500 .ANG.. Because of the concavity of plug surface and its protrusion above the insulation layer, only a rough surface can be formed. Subsequently, when a photolithographic operation is carried out to pattern out conductive lines, light will likely be defocused since the surface is rough. Consequently, shapes and positions of those conductive lines will be somewhat affected and product quality will be compromised.
An alternative method is to polish the tungsten layer using CMP at the very start, and then perform a CMP touch-up operation at a slower polishing speed thereafter. This method is able to obtain a highly planar tungsten plug surface with a maximum variation within a range of about 200 .ANG. to 300 .ANG.. However, the cost can be as high as US$ 35 per silicon wafer.
In light of the foregoing, there is a need to provide a method of manufacturing a metal plug that can lower the cost of production.